US intelligence officials have said they believe pro-Russian separatists most probably shot down the Malaysia Airlines plane MH17 "by mistake" last week, not realising it was a civilian passenger flight.
Speaking on condition of anonymity in Washington, they said the "most plausible explanation" for the destruction of the plane was that the separatists fired a Russian-made SA-11 missile at it after mistaking it for another kind of aircraft.
"Five days into it, [the crash] it does appear to be a mistake," one of the officials told reporters on Tuesday.
They said that their assessments were backed up by evidence from social media and by intercepted conversations of known pro-Russian separatists, whose voice prints had been verified by US agencies.
The speakers initially bragged about shooting down a transport plane, but later acknowledged that they might have made a mistake, the officials said.
President Barack Obama's administration has said it is convinced the airliner was brought down last Thursday by an SA-11 ground-to-air missile fired from territory in eastern Ukraine controlled by pro-Russian separatists.
Although the US had observed a flow of heavy weapons, including air defence systems, into Ukraine from Russia, intelligence agencies had not seen the larger SA-11 missiles being moved into the country before the airliner was shot down, the officials said.
The Russian military had been training the rebels at a large base in Rostov on various weapons, including air defence systems, the officials said, but there was no explicit evidence of the Russians training the separatists on the SA-11 missile batteries.
Day of mourning
A train bearing the dead from the downed plane finally reached Ukrainian government-held territory on Tuesday, but the pro-Russian separatists in control of the crash site showed little willingness to allow the full-scale investigation demanded by world leaders.
Five days after the plane was blown out of the sky, refrigerated railway cars bearing victims' bodies, gathered up after several days in the sun, rolled out of the war zone and into a weedy railyard in the city of Kharkiv.
The dead will be flown to the Netherlands, the homeland of most of the victims, for identification.
The Dutch government declared Wednesday a day of national mourning as the country prepared for the arrival of the first bodies in the afternoon.
It was unclear how many of the 282 corpses reported found so far were on the train.
The crash killed all 298 people aboard.